I've reported this before and it was "fixed" but I don't think I made it clear what I meant. 

https://tracker.phpbb.com/browse/PHPBB-15766

For clarity I'll use "No" when referring to yes, "No" or never. 

Suppose you have a private forum and the admin has only added the  admin and moderator groups with roles assigned to each. It's a simple way to restrict access to a forum. The other groups have no permissions, there is no access because it defaults to "No". 

I you go to Forum permissions >> Check "All forums" and submit

Since we have selected all forums all groups with any permissions in any forum are listed under "Manage groups". Check "Select all groups" and click edit permissions. 

The next page will present you with a page listing all forums and the permissions set for each group which is extremely useful because you have complete overview of everything.

However "No role assigned" can mean two things.

(1)The group has not been added to the forum and no permissions are set. 

(2)Advanced permissions are used and there could be yes permissions. 

When a group has not been added there should be indication "Group not added to this forum, permissions  not set, defaults to all "No" permissions".

If advanced permissions are used there should be indication "Advanced permissions set".